Shadowgraph method has been proposed for fast and noninterference measurement of ultrasonic pressure field. However, special care has been needed in choosing an appropriate optical propagation length to obtain a satisfactory signal-to-noise ratio of the measurement while avoiding error from a geometrical optics approximation. In this study, we propose a new numerical method replacing the geometrical optics approximation to retrieve the optical phase for the measurement. Optical intensity distribution obtained from the numerical simulation of optical propagation based on the Huygens-Fresnel principle agreed well with the measurement at a relatively large optical propagation length at which the geometrical optics approximation failed. Optical phase retrieval from the simulated optical intensity distribution by the proposed method was then tested. The range of optical propagation for successful phase retrieval was extended a few times by the proposed method compared with the geometrical optics approximation.